This invention relates in general to the field of photolithography and, more particularly, to a method for correcting pattern errors on a photomask.
As feature sizes for integrated circuits (ICs) decrease, semiconductor manufacturers face new challenges controlling feature sizes during processing. Processing steps that can affect feature size control include lithography, resist development and etching. The industry currently uses many different techniques to control feature size on a wafer. For example, optical proximity correction (OPC) of the pattern on a photomask can improve the feature size control during lithography, while chemical mechanical polishing (CMP) can improve control after etching is completed.
A photolithography system typically uses a photomask, also known as a reticle or mask, to transfer a pattern representing an IC onto a semiconductor wafer. The photomask is formed from a photomask blank by using a standard photolithography process, such as laser or electron beam lithography. During the lithography process, optical proximity effects may produce variations in the dimensions of a feature as a function of the proximity of other nearby features. A common optical proximity effect, known as iso-dense bias, creates a difference between the printed dimensions of an isolated feature and a dense feature having the same design dimensions.
Semiconductor manufacturers typically attempt to correct iso-dense bias on the wafer by correcting the pattern on the photomask. For example, U.S. Pat. No. 5,208,124 issued to Sporon-Fiedler, et al., discloses a technique for modifying a photomask pattern based on one or more equations that define the deviation of a wafer image from the pattern in a design data file. In this case, iso-dense bias is corrected by adjusting the feature sizes on the photomask based on line width and pitch. Also, U.S. Pat. No. 5,867,253 issued to Nakae discloses a method of determining the local pattern density within a certain radius of a design point and comparing it to a reference pattern density in order to determine if correction is required. Semiconductor manufacturers also may correct proximity effects on the wafer by changing the exposure dose for features in isolated and dense areas. For example, U.S. Pat. No. 5,657,235 issued to Liebmann, et al. discloses a method of creating pattern data with relative dose information. Feature sizes and shapes on a wafer are modified by assigning exposure doses for each feature, rather than physically manipulating the feature sizes on the mask. These techniques reduce iso-dense bias on the wafer but fail to solve the iso-dense bias effect produced during fabrication of the photomask.
In accordance with the teachings of the present invention, disadvantages and problems associated with correcting pattern errors on a photomask have been substantially reduced or eliminated. In a particular embodiment, a method for correcting feature size errors on a photomask is disclosed that modifies a dimension of a second feature based on a calculated total line edge length for a first feature.
In accordance with one embodiment of the present invention, a method for correcting pattern errors on a photomask includes determining a proximity effect caused by a first feature on a second feature in a pattern data file. The method calculates a total line edge length for the first feature and modifies a dimension of the second feature based on the total line edge length calculated for the first feature.
In accordance with another embodiment of the present invention, a method for fabricating a photomask includes determining a proximity effect caused by one or more first features on a second feature in a first pattern data file. The method calculates a total line edge length by summing edge lengths of the one or more first features and modifies an edge position of the second feature based on the total line edge length calculated for the one or more first features. The method further generates a second pattern data file including the one or more first features and the modified second feature. A photoresist layer of a photomask blank is patterned using the second pattern data file to form one or more first regions and a second region in an opaque layer that are respectively substantially equal in size to the one or more features and the second feature in the first pattern data file.
In accordance with a further embodiment of the present invention, a photomask includes a patterned opaque layer formed on a transparent substrate. The opaque layer is generated with a modified pattern file that includes data representing a first feature and a second feature modified based on a total line edge length of the first feature.
Important technical advantages of certain embodiments of the present invention include a photolithography technique that corrects iso-dense bias effects on a photomask. Most semiconductor manufacturers ignore errors on the photomask caused by iso-dense bias when manufacturing the photomask. Traditionally, manufacturers compensate for the iso-dense bias effect on the photomask indirectly by compensating the mask pattern for iso-dense bias on the wafer. This method is usually not sufficient to completely compensate for the photomask iso-dense bias effects that occur on the wafer. The present invention provides a method for directly compensating for iso-dense bias on the photomask by modifying dense features in a pattern data file and fabricating the photomask with the modified features.